Boring sidebar: header controls and a per-project thread limit

[diogovalada]

Project folders are a welcome improvement. Two controls I used in the legacy sidebar are still missing:

• A menu in the sidebar header for grouping and sorting, instead of going into Settings to change the view.
• A visible-thread limit per folder: show the first N threads, then "Show more". A busy project can otherwise push all the others far down the sidebar. The open thread should stay visible even if it's past the limit.

These were points 2 and 5 of my comment on #21039. Would maintainers be open to restoring these controls? They could be separate changes.

The other requests already have their own threads: sorting choices in #6964, broken manual project ordering in #27062, and compact rows in #17319. This discussion is just for the two controls above.